126.96.36.199 should be considered really more or less a kind of test DNS server.
188.8.131.52 and 184.108.40.206 point to the load balancers of Google's public DNS. Since these are in truth not a SOA for any domain (and never will be by design). I think of them as perfect forwarders since they will only forward themself. They are also pingable and - as most of the things operated by Google - near perfectly reliable. The DNS servers from my ISPs are not nearly as fast and available. See Google's statement